COURSE TITLE: MEDICAL BIOCHEMISTRY
TOPIC: PROTEIN METABOLISM
LECTURER: MR. BELLO T. O.
Among the classifications in amino acids, there
is a form of classification that is based on their
metabolism where there are glucogenic and
ketogenic amino acids
Glucogenic amino acids yield the precursors (keto
acids) for the synthesis of carbohydrates i.e.
arginine, phenylalanine e.t.c
Ketogenic amino acids yield the precursors
(ketone compounds like acetone,acetoacetate &
β-hydroxyl butyric acid )for the synthesis of
biomolecules like lipids. i.e leucine
OVERVIEW OF AMINO ACIDS METABOLISM
Metabolism has two parts; catabolism and
anabolism
Catabolism of amino acids
Degradation of amino acids with some of the rxns.
(oxidative deamination, transamination, decarboxylation)
to give Keto acids (carbon skeleton) and ammonia
Fates of carbon skeleton - gluconeogenesis, non
essensial amino acids
Fates of ammonia - Urea cycle for its excretion,
assimilation into keto acids for non essential amino acids
synthesis.
Why is ammonia toxic?
Metabolism of some amino acids, their
specialized products and their metabolic
disorder.
Glycine
1. Synthesis of glycine:
Synthesized from serine in the presence of
hydroxymethyl transferase which is tetrahydrofolate dependent.
Can be obtained from threonine, catalysed by
threonine aldolase
Glycine can be synthesized from one carbon
compound (N5N10-methylene THF),CO2 and NH3
in the precence of glycine synthase.
2. Its catabolic product can actively be
involved
in the synthesis of many specialized product
like Heme, purines, creatine e. t. c.
SYNTHESIS OF SPECIALIZED PRODUCTS FROM
GLYCINE
Formation of purine ring: utilized in the
formation of position 4 and 5 of carbon and
position 7 nitrogen in the purine ring.
Synthesis of heme (porphyrine ): glycine
condenses with succinyl CoA to form δ-amino
levulinate which is the precursor for Heme
synthesis.
Succinyl CoA +glycine -δ-amino levulinate (ALA)
Biosynthesis of creatine:glycine, methionine and
arginine are required for the synthesis of
creatinine
Transfer of guanidino group of arginine to
glycine in the presence of arginine transamidase to
produce guanidoacetate (glycocyamine).
S-Adenosylmethionine (active methionine)
transfers methyl group to glycocyamine to produce
creatine which occurs in the liver.
Creatine is phosphorylated to phosphocreatine
(creatine phosphate) in the presence of creatine
kinase and stored in the muscles as a high energy
compound.
Metabolic disorders of glycine metabolism:
Glycinuria: This is a defect in which
excretion of glycine is above 0.5 – 1 g/day
due to defective renal reabsorption
Primary hyperoxaluria:
This defect is due to glycine transaminase
impairment in glyoxalate oxidation to
formate. It is charactcetrized by increased
urinary oxalate leading to oxalate stones
(oxalosis) Deposition of oxalate is observed
in various tissues.
PHENYLALANINE AND TYROSINE :
Metabolism of phenylalanine occurs through
tyrosine
Tyrosine can be incorporated into various
proteins like epinephrine, norepinephrine,
dopamine (catecholamine), thyroid hormones
and pigments like melanin.
Conversion of phenylalanine to tyrosine:
Degradation of phenyalanine occurs through
tyrosine. Phenylalanine is hydroxylated by
phenylalanine hydroxylase (present in the liver)
to produce tyrosine. This is irreversible reaction
and requires biopterine as the coenzyme.
(tetrahydrobiopterine) which is oxidised to
dihydrobiopterine, then regenerated by NADPH
dependent dihydrobiopterine reductase. The
failure or blockage of phenyalanine hydroxylase
may result in phenylketonuria.
Tyrosine undergoes transamination to give phydroxyphenylpyruvate in the presence of tyrosine
transaminase (PLP dependent).
p-hydroxyphenylpyruvate undergoes oxidative
decarboxylation in presence of p-hydroxyphenylpyruvate oxidase or dioxygenase (copper
containing enzyme) to produce homogentisate
Homogentisate is converted to 4-maleylacetoacetate a reaction catalysed by homogentisate
oxygenase which requires molecular oxygen to
break the aromatic ring.
Isomerization of 4-maleylacetoacetate to 4
fumarylacetoacetate reaction catalysed by
maleylacetoacetate isomerase.
4 fumarylacetoacetate undergoes hydrolysis to
yield fumarate and acetoacetate which are the
precursor in lipid synthesis, TCA cycle and
glucose synthesis.
SYNTHESIS OF SPECIALIZED PRODUCTS FROM
TYROSINE
1. MELANIN:
Synthesis of melanin occurs in the melanosomes
present in the melanocytes.
Tyrosine is the precusor for melanin and
tyrosinase (copper containing) is the primary
enzyme that is involved in the synthesis of the
pigment.
Tyrosine gets converted to 3,4 –
dihydroxyphenylalanine (DOPA) in the reaction
catalysed by tyrosinase
. DOPA gets converted to dopaquinone and in
subsequent reactions it forms leucodopachrome,
followed by 5,6 dihydroxyindole. Oxidation
process takes place, Tyrosinase is involved in the
sequential enzymatic conversion of 5,6
dihydroxyindole to indole quinone.
Melanochrome is synthesized from indole
quinone which on polymerization produces
melanin.
Alternatively, dopaquinone is condensed with
cysteine and red melanin is generated.
2. BIOSYNTHESIS OF THYROID HORMONES:
Thyroid hormones are synthesized from tyrosine.
Tetraiodothyronine (thyroxine) and
triiodothyronine hormones.
Iodinization of tyrosine ring occurs to produce
mono and diiodotyrosine from which
triiodothyronine (T3) and thyroxine (T4) are
synthesized
Protein thyroglobulin undergoes proteolytic
breakdown to release the free T3and T4
hormones.
Disorders of tyrosine and phenylalanine
metabolism:
1. PHENYLKETONURIA:
Due to defective enzyme phenylalanine
hydroxylase, this leads to accumulation of
phenylalanine, which by transamination is
converted to phenylpyruvate in tissues and
blood. Elevated amount of phenylpyruvate and
other keto acids are then found in urine.
Biochemical manifestation:
(i) Effect on CNS
Mental retardation, failure to walk, failure to
grow, tremor and failure to transport other
aromatic amino acid like tryptophan and tyrosine
and this leads to defect in myelin formation.
(ii) Effect on pigmentationInhibition of tyrosinase leads to albinism
Treatment:
Dietary intake of phenylalanine should be measured
in plasma levels and adjusted.
Diagnosis:
Done by Guthrie test and ferric chloride test and
green colour is obtained. Normal values in
phenylalanine in plasma PKU 20-65mg/dl
2. TYROSINEMIA TYPE II: (RICHNER-HANART
SYNDROME)
Due to blockage of tyrosine transaminase
hence accumulation and excretion of tyrosine
and its metabolites
Characterised in skin (dermatitis) and eye
lession
ALKAPTONURIA:
Defect on the enzyme homogentisate oxidase,
hence accumulation of homogentisate in blood
and tissues. Alkapton is the pigment produced,
in case of accumulation occurs in the connective
tissues, bones and various organs resulting in
ochronosis
Diagnosis:
Benedict’s test,carry out ferric chloride and
silver nitrate test in urine
Treatment:
Consumption of proteins with low phenylalanine
contents
TYOSINOSIS OR TYROSINEMIA TYPE I:
Due to deficiency of Fumarylacetoacetate
hydroxylase and maleylacetoacetate isomerase
which may lead to liver failure, rickets, renal
tubular dysfunction and polyneuropathy
Treatment:
Recommended diets with low tyrosine,
phenylalanine and methionine
ALBINISM:
Causes:
Deficiency or lack of enzyme tyrosinase
Decrease in melanosomes of melanocytes
Impairment in melanin polymerization
Lack of protein matrix in melanosomes
Limitation of substrate (tyrosine) availability
Presence of inhibitors of tyrosinase
The common cause of albinism is defect on
tyrosinase, enzyme responsible for the synthesis
of melanin.
Clinical manifestation:
Melanin protect the body from sun rays hence
albinos are sensitive to sunlight and susceptible
to skin cancer (carcinoma). Photophobia is
associated with lack of the pigment in the eyes.
UREA CYCLE:
Site of metabolism is liver and urea is the end
product of protein metabolism as elucidated by
Hans kreb and Kurt Henseleit in 1932
Urea has two amino groups, one from ammonia
and other from aspartate and carbon is supplied
from carbon dioxide.
Enzymes involved:
Two enzymes are found in mitochondria while
others are cytosolic.
Steps in urea cycle:
Synthesis of carbamoyl phosphate:
Synthesized by condensation of carbon dioxide
(CO2) with ammonium ions (NH4+) to form
carbamyl phosphate in the presence of carbamoyl
phosphate synthetase, a rate limiting enzyme
requiring N-Acetylglutamate.
Formation of citrulline:
Citrulline is synthesized when ornithine is
condensed with carbamoyl phosphate in the
presence of ornithine transcarbamoylase. The
citrulline synthesized at this point is transported
into cytosol. Ornithine and citrulline are basic
amino acids.
Synthesis of arginosuccinate:
Citrulline is condensed with aspartate in the
presence of arginosuccinate synthetase to yield
arginosuccinate. This is a step for second amino
group incorporation and ATP is required and is
cleaved to yield AMP and Ppi
Cleavage of arginosuccinate:
Arginosuccinate is cleaved by arginosuccinase to
give fumarate and arginine. Arginine is immediate
precursor for urea synthesis. Fumarate enters into
TCA cycle, gluconeogenesis.
Formation of urea:
Arginase cleaves arginine to yield urea and
ornithine. Ornithine enters into mitochondria for
re-use. Arginase is activated by Mn2+
UREA CYCLE
CLINICAL SIGNIFICANCE OF UREA
A moderately active man consuming about 300
gm carbohydrates, 100 gm of fats and 100 gm of
proteins daily must excrete about 16.5 gm of N
daily. 95 per cent is eliminated by the kidneys and
the remaining 5 percent, for the most part as N,
in the faeces.
1. Normal Level
The concentration of urea in normal blood plasma
from a healthy fasting adult ranges from 20 to
40mg%.
2. Increase of Levels
Increases in blood urea may occur in a number of
diseases in addition to those in which the kidneys
are primarily involved.
The causes can be classified as:
Prerenal
Most important are conditions in which plasma vol/
body-fluids are reduced:
Salt and water depletion,
Severe and protracted vomiting as in pyloric and
intestinal obstruction,
Severe and prolonged diarrhoea,
Pyloric stenosis with severe vomiting,
Haematemesis,
Haemorrhage and shock; shock due to severe
burns,
Ulcerative colitis with severe chloride loss,
In crisis of Addison’s disease (hypoadrenalism).
Renal
The blood urea can be increased in all forms of
kidney diseases:
In acute glomerulonephritis.
In early stages of type II nephritis (nephrosis) the
blood urea may not be increased, but in later
stages with renal failure, blood urea rises.
Other conditions are malignant nephrosclerosis,
chronic pyelonephritis and mercurial poisoning.
In diseases such as hydronephrosis, renal
tuberculosis; small increases are seen but
depends on extent of kidney damage.
Postrenal Diseases
These lead to increase in blood urea, when there
is obstruction to urine flow. This causes retention
of urine and so reduces the effective filtration
pressure at the glomeruli; when prolonged,
produces irreversible kidney damage.
Causes:
Enlargement of prostate,
Stones in urinary tract,
Stricture of the urethra,
Tumours of the bladder affecting urinary flow.
3. Decreased levels:
Decreases in blood urea levels are rare. It may
be seen:
In some cases of severe liver damage,
Physiological condition: Blood urea has been
seen to be lower in pregnancy than in normal
nonpregnant women.
REGULATION OF UREA CYCLE:
The rate limiting enzyme is carbamoyl phosphate
synthetase I. Requires NAG (N-acetyl glutamate)
synthesized from acetyl CoA and Glutamate.
Increase of NAG increases synthesis of urea in the
liver.
Carbamoylphosphate synthetase I and Glutamate
dehydrogenase located in the mitochondria
coordinate the synthesis of NH3 for the synthesis
of carbamoyl phosphate.
The remaining four enzymes particitipate in the
synthesis of urea depend on the concentration of
the substrates.
INTEGRATION OF UREA CYCLE WITH TCA CYCLE:
The formation of fumarate in urea cycle is the
integrating point to TCA cycle.
Oxaloacetate undergoes transamination to
produce aspartate which enters urea cycle as
the second source of amino group to synthesis
urea.
ATP (12) generated in the TCA cycle while 4
ATP are utilized in urea cycle.
CO2 and H2O are the end product that are
formed on complete oxidation of various
metabolites whereby CO2 generated is utilized
in the urea synthesis.
Disorder
Enzyme involved
Hyperammonemia type 1
Carbamoyl
phosphate
synthetase I
Hyperammonemia type II
Ornithine
transcarbamoylase
Citrullinemia
Arginosuccinate synthase
Arginosuccinemia
Arginosuccinase
Hyperargininemia
Arginase
Blood urea clinical significance:
Pre-renal: is associated with increased protein
breakdown hence negative nitrogen balance
observed during diabetic coma, thyroxicosis,
leukemia bleeding disorders
Renal-increased: in cases of patients suffering
from
acute
glomerulonephritis,
chronic
nephritis, nephrosclerosis,polycystic kidney.
Post renal-elevated: in cases of urinary tract
obstruction i.e. tumor, stones
THE END